1. Field of the Invention
This invention relates to a method of determining, on the basis of a histogram of image signals carrying an image recorded on a recording medium such as a stimulable phosphor sheet, a desired image signal range, that is, a range of the image signals corresponding to a part of the image which is necessary for a diagnosis or the like.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to a radiation passing through an object such as the human body to have a radiation image of the object stored thereon, and is then exposed to stimulating rays such as a laser beam which cause the stimulable phosphor sheet to emit light in proportion to the stored radiation energy. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, image processing is carried out on the electric image signal, and the radiation image of the object is reproduced as a visible image by use of the processed image signal on a recording medium such as a photographic film, a display device such as a cathode ray tube (CRT), or the like.
There are many cases where only a part of the recorded image is necessary. For example, in the case of recording of the human cervical region, the recorded image is as shown in FIG. 3. In the recorded image, only image information on the cervical vertebrae A and the soft part around the cervical vertebrae is generally necessary and image information on the other parts, that is, the empty image portion B where the radiation directly impinges upon the stimulable phosphor sheet and the image signal level is the highest, the chin portion C and the shoulder portion D, are not necessary. In such a case, it is preferred that the whole image be not reproduced as a visible image but only a part of the image corresponding to the cervical vertebrae A and the soft part therearound which is necessary for the diagnosis be selectively reproduced within a predetermined density range suitable for viewing for a given diagnostic purpose, thereby improving the contrast and resolution.
In accordance with the method proposed in U.S. Pat. No. 4,527,060 in order to satisfy the demand, a preliminary read-out (A read-out operation for ascertaining the image information of a radiation image stored on the stimulable phosphor sheet which is carried out in advance by use of stimulating rays having stimulation energy of a level lower than the level of the stimulation energy of stimulating rays used in a final read-out operation for obtaining a visible image for viewing, particularly for diagnostic purposes, as described in U.S. Pat. No. 4,527,060 is first carried out and a histogram of the image signals (image signal level) obtained by the preliminary read-out is then obtained. At the same time, the maximum image signal level Smax and the minimum image signal level Smin of a desired image signal range in the histogram are calculated, and the read-out conditions for the final read-out are adjusted so that the maximum image signal level Smax and the minimum image signal level Smin correspond respectively to the maximum signal level Qmax and the minimum signal level Qmin of a desired input signal range in an image processing means which are determined by the maximum density Dmax and the minimum density Dmin of the correct density range in the reproduced visible image. Thereafter said final read-out is carried out on the basis of the read-out conditions thus determined.
Further, it may be possible to satisfy the demand by obtaining a histogram of the image signals (image signal level) obtained by the preliminary read-out, calculating the maximum image signal level Smax and the minimum image signal level Smin of a desired image signal range in the histogram, adjusting the gradation processing conditions so that the maximum image signal level Smax and the minimum image signal level Smin correspond respectively to the maximum signal level Qmax and the minimum signal level Qmin of a desired input signal range in an image reproducing means (visible image output means) which are determined by the maximum density Dmax and the minimum density Dmin of the correct density range in the reproduced visible image and carrying out the gradation processing according to the gradation processing conditions thus determined.
In this method, the image signals obtained by the final read-out may be used instead of the image signals obtained by the preliminary read-out. Also in this case, a histogram of the image signals are first obtained and then said Smax and Smin are calculated and the gradation processing conditions are determined so that the Smax and Smin respectively correspond to said Rmax and Rmin.
By the term "read-out conditions" as used herein are meant various conditions affecting the relationship between the input (quantity of light emitted by the stimulable phosphor sheet upon stimulation) and the output (electric image signal level) of the photoelectric read-out means, e.g., the read-out gain (sensitivity) determining the relationship between the input and the output, the scale factor (latitude) and the level of the stimulating used in the read-out.
By the term "image processing conditions" as used herein are meant various conditions affecting the relationship PG,6 between the input to the image processing means and the output thereof, for example, gradation processing conditions and frequency response processing conditions.
As mentioned above, the level of the stimulating rays used in the preliminary read-out is adjusted to be lower than the level of the stimulating rays used in the final read-out. That is, the effective energy of the stimulating rays which the stimulable phosphor sheet receives per unit area in the preliminary read-out is adjusted to be lower than the effective energy of the stimulating rays used in the final read-out.
In order to carry out the methods described above, the desired image signal range, that is, the Smax and Smin must be appropriately determined. However, in the method disclosed in Japanese Unexamined Patent Publication No. 60(1985)-156055, the desired image signal range in a given histogram is roughly determined referring to the pattern of the histogram which depends upon the image recording portion of the object and/or the image recording method to a certain extent, and the Smax and Smin within the desired image signal range are determined on the basis of an appropriate frequency threshold value. Accordingly, the values of the Smax and Smin obtained in accordance with the method are only just appropriate and cannot be sufficient.
That is, the image signal histogram of the whole human cervical region image is as shown in FIG. 1, and if the desired image portion includes the chin and shoulder portion I, the cervical vertebrae II and the soft part III such as skin, and occupies almost the whole area of the histogram except the empty image portion IV which can be clearly distinguished on the basis of the form of the histogram, that is, if no unnecessary image portion exists on the lower level side of the desired image portion, the maximum image signal level Smax and the minimum image signal level Smin of the desired image signal range carrying the desired image portion can be substantially appropriately determined by the use of an appropriate frequency threshold value set on the basis of the form of the histogram. However, in the case that the desired image portion includes only the cervical vertebrae II and the soft part III and unnecessary image portion such as the chin and shoulder portion I exists on the lower level side of the desired image portion, it is difficult to appropriately determine the Smax and Smin by such a method in which the Smax and Smin are simply determined from the histogram of the whole image by the use of a predetermined frequency threshold value.